In PET imaging, or positron emission tomography, a radiopharmaceutical agent is administered, via injection, inhalation and/or ingestion, to a patient. The physical and bio-molecular properties of the agent then concentrate at specific locations in the human body. The actual spatial distribution, intensity of the point and/or region of accumulation, as well as the kinetics of the process from administration and capture to eventual elimination, all have clinical significance. During this process, the positron emitter attached to the radiopharmaceutical agent emits positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc. Each positron interacts with an electron of the object, is annihilated and produces two gamma rays at 511 keV (electron-positron annihilation event), which travel at substantially 180 degrees apart. The two gamma rays then cause a scintillation event at a scintillation crystal of the PET detector, which detects the gamma rays thereby. By detecting these two gamma rays, and drawing a line between their locations or “line-of-response,” the likely location of the original annihilation is determined. While this process only identifies one line of possible interaction, accumulating a large number of these lines, and through a tomographic reconstruction process, the original distribution is estimated with useful accuracy. In addition to the location of the two scintillation events, if accurate timing—within a few hundred picoseconds—is available, time-of-flight calculations are also made in order to add more information regarding the likely position of the annihilation event along the line. Limitations in the timing resolution of a scanner determine the accuracy of the positioning along this line. Limitations in the determination of the location of the original scintillation events determine the ultimate spatial resolution of the scanner. A specific characteristic of the isotope (for example, energy of the positron) contributes (via positron range and co-linearity of the two gamma rays) to the determination of the spatial resolution for a specific radiopharmaceutical agent.
As timing accuracy is an important factor, timing differences due to electronic pathway disparities need to be considered and addressed. The conventional way of measuring differences due to electronic pathway disparities includes using a signal generated from an external source such as a radioactive isotope. Thus, a radioactive source with phantom is typically used to measure the electronic pathway timing disparities. When processing the obtained data, an iterative method is used to perform measurements due to the indirect way in which the timing is measured. This iterative method is time consuming and costly.
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present embodiments.